1. Field of the Invention
The present invention relates to a device for scanning the original plate of a microfilm copying machine on which an aperture card is loaded.
2. Description of the Prior Art
An original plate scanning device for a microfilm copying machine must carry out a scanning operation at a constant speed which corresponds to the speed of rotation of a photo-sensitive member and to the projection magnification. In other words, when the required scanning speed is different from the average or DC speed component of the actual speed, or when the actual speed includes an AC component (namely, a slight variation), an accurate, high resolution copying operation cannot be achieved.
It is technically possible to allow the average speed component of the actual speed, which is one of the factors lowering the resolution, to approach the necessary speed; however, the conventional means for reducing the AC component of actual speed (i.e., the slight variation) is poor in performance and high in cost.
More specifically, the slight variation of the actual speed is attributable to the drive section of the scanning device, or to the vibration of the copying machine body which is transmitted to the scanning device. In order to reduce the speed variation, the following conventional means is employed by making reference to FIG. 1: A brake device is provided on the drive side of the scanning device so that a brake surface 16 is formed on a drive shaft 14 which drives wire means 12. The wire means 12 can be a drive steel tape or a wire rope which is fastened to one end of an original plate 10. A brake pad 20 of a brake bracket 18 is elastically biased to press against the brake surface.
Alternatively, referring to FIG. 2, a brake device has a leaf spring 26 which is held by a bracket 24 and secured to an original plate 10, and the leaf spring thus held pushes a brake pad 28 against a guide shaft 22 which is adapted to guide the original plate 10.
In yet a further prior art device, a commercially available torque damper is added to the drive shaft 14 of the scanning device. However, in the case in which the brake is provided on the drive or driven side, it is difficult to generate a constant brake force and to maintain it. Even when it is possible to do so, the adjustment is considerably troublesome, and, when it is intended to make the adjustment simple, the brake device becomes expensive. Similarly, the torque damper is also relatively expensive.